Method for compensating for variations in structures of an integrated circuit

ABSTRACT

A method of for compensating for variations in structures of an integrated circuit. The method includes (a) selecting a mask design shape and selecting a region of the mask design shape; (b) applying a model-based optical proximity correction to all of the mask design shape; and after (b), (c) applying a rules-based optical proximity correction to the selected region of the mask design shape.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/412,725 filed on Mar. 6, 2012 which is a continuation of U.S. patentapplication Ser. No. 12/208,521 filed on Sep. 11, 2008, now U.S. Pat.No. 8,176,446, issued May 8, 2012.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuitmanufacture; more specifically, it relates to a method for compensatingfor variations in structures of an integrated circuit.

BACKGROUND OF THE INVENTION

Despite the use of model-based optical proximity correction (OPC) inadvanced integrated circuit fabrication processes, uncorrected shapevariations still occur that result in integrated circuit structuresvariations that adversely impact integrated circuit performance.Accordingly, there exists a need in the art to mitigate the deficienciesand limitations described hereinabove.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a method, comprising: (a)selecting a mask design shape and selecting a region of the mask designshape; (b) applying a model-based optical proximity correction to all ofthe mask design shape; and after (b), (c) applying a rules-based opticalproximity correction to the selected region of the mask design shape.

A second aspect of the present invention is a method, comprising: (a)selecting a mask design shape and selecting a regions of the mask designshape, the mask design shape comprising a spine and two or more fingersextending perpendicularly from the spine, the selected regionsconsisting of regions of each of the fingers that are proximate to thespine; (b) applying a model-based optical proximity correction to all ofthe mask design shape; and after (b), (c) applying a rules-based opticalproximity correction to the selected region of the mask design shape.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of an illustrative embodiment when readin conjunction with the accompanying drawings, wherein:

FIG. 1A is a top view and FIGS. 1B and 1C are side views through lines1B-1B and 1C-1C respectfully of an ideal integrated circuit;

FIG. 1D is a top view of an actual integrated circuit similar to that ofFIGS. 1A, 1B and 1C illustrating a defect to be corrected by theembodiments of the present invention;

FIG. 1E is a top view of a compensated mask shape according toembodiments of the present invention that mitigate the defectillustrated in FIG. 1D;

FIG. 1F is a detailed view of FIG. 1E;

FIG. 1G is a top view of an actual integrated circuit illustratingelimination/reduction of the defect to be corrected by the embodimentsof the present invention;

FIG. 2A is a top view illustrating an ideal and a defective multi-fingerfield effect transistor (FET) structure;

FIG. 2B is a top view of a gate electrode photomask shape correspondingto the FET of FIG. 2A after model-based OPC;

FIG. 2C is a top view of a gate electrode photomask shape correspondingto the FET of FIG. 2A after model-based OPC and rule-based OPC accordingto embodiments of the present invention;

FIG. 3 is a flowchart of a first method for compensating for variationsin structures of an integrated circuit;

FIG. 4 is a flowchart of a second method for compensating for variationsin structures of an integrated circuit;

FIG. 5 is a flowchart of a third method for compensating for variationsin structures of an integrated circuit; and

FIG. 6 is a schematic block diagram of a general-purpose computer forpracticing the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A mask design shape is defined as a data element in a shapes file usedto fabricate photomasks. A mask shape is defined as a region of definedopacity to actinic radiation as opposed to regions of definedtransparency on a photomask used to fabricate an integrated circuit.Each mask shape on a photomask corresponds to a mask design shape in theshapes file. A circuit shape is defined as a physical structure (otherthan photoresist) formed on an integrated circuit during fabrication ofthe integrated circuit. Circuit shapes correspond to mask shapes.Fabrication of an integrated circuit requires multiple process steps,many process steps require one or more photomasks (mask levels), andeach photomask requires a shapes file. Mask shapes on a photomask aretransferred to an integrated circuit using a photolithographic processwhich includes: exposing a photoresist layer on the integrated circuitto actinic radiation through a photomask, developing the photoresistlayer to transfer the pattern of mask shapes to the photoresist layer,performing a fabrication process (e.g., an etch, deposition, ionimplantation) and then removing the photoresist layer. Actinic radiationis defined as radiation of a particular wavelength (e.g., UV) thatcauses a chemical reaction in a photoresist layer during aphotolithographic process.

Model-based OPC operates using a computer model of the photolithographicprocess. The model determines how simulated mask shapes would appear asa simulated pattern in a photoresist layer after performing thephotolithography process. When the simulated photoresist pattern doesnot fall within specified locations, the simulated mask shapes are thenadjusted until the simulated the simulated photoresist pattern does fallwithin specified locations. The mask design shapes are then adjusted toconform to the adjusted simulated mask shapes. The OPC corrected maskdesign shapes are then used to fabricate a photomask having OPCcorrected mask shapes. Rule-based OPC tests for a particular conditionoccurring in mask design shapes of a particular mask level and thenapplies corrections to those design shapes based on rules. For example,a ‘nested’ line consists of mask shapes on the photomask which employthe minimum allowed line and space. An exemplary rule of a rule-basedOPC for nested lines with 100 nm minimum line and 100 nm minimum spacecould be to remove 5 nm per edge from the line so that the line is 90 nmwide and the space is 110 nm wide.

FIG. 1A is a top view and FIGS. 1B and 1C are side views through lines1B-1B and 1C-1C respectfully of an ideal integrated circuit. In FIG. 1A,an ideal circuit shape 100 crosses an interface 105 between anunderlying first region 110 and an abutting and underlying second region115. As illustrated in FIG. 1A, circuit shape 100 crosses interface 105perpendicularly (i.e. at a right angle to). Interface 105 may define adifference in reflectivity to actinic radiation between first region 110and second region 115, a difference in height between first region 110and second region 115, or both a difference in reflectivity to actinicradiation and height between first region 110 and second region 115. InFIG. 1B, top surface 120 of first region 110 is coplanar with topsurface 125 of second region 115, but the reflectivity of the first andsecond regions are different. In FIG. 1C, first region 110 has a heightH1 while second region 115 has a height H2 where H2 is greater than H1.Alternatively, height H2 could be less than height H1. A top surface125A of second region 115 is raised above a top surface 120A of firstregion 110 forming a step 127. Top surface 120A of first region 110 andtop surface 125A of second region 115 are parallel but not coplanar. InFIG. 1C, a top surface 120A of first region 110 may have a samereflectivity to actinic radiation or a different reflectivity to actinicradiation from second region 115. In one example, first and secondregions 110 and 115 comprise different materials (e.g., silicon andinsulator; metal and insulator).

FIG. 1D is a top view of an actual integrated circuit similar to that ofFIGS. 1A, 1B and 1C illustrating a defect to be corrected by theembodiments of the present invention. FIG. 1D is similar to FIG. 1Aexcept an actual circuit shape 110A has a necked region 130 proximate tointerface 105. Necked region 130 extends over both first and secondregions 110 and 115 distances D1 and D2 respectively. In one example D1and D2 are about the same. In another example D1 and D2 are different.Necked region 130 occurs when a reflectivity difference to actinicradiation between first and second regions 110 and 115 exceeds athreshold value, when a height difference between first and secondregions 110 and 115 exceeds a threshold value, or when both thereflectivity difference to actinic radiation and the height differencebetween first and second regions 110 and 115 exceed a combination ofrespective threshold values. Necked region 130 necks laterally distancesD3 and D4. In one example D3 and D4 are about the same. In anotherexample D3 and D4 are different. Due to process and other variabilitysources, the combined necking D3 and D4 could range from 0% (no neckingat all) to 100% of the width of circuit shape 100A (an open) dependingwhere circuit shape 100A is located on a integrated circuit chip and/orwhere the integrated chip is located on a wafer having an array multipleintegrated circuit chips.

FIG. 1E is a top view of a compensated mask shape according toembodiments of the present invention that mitigate the defectillustrated in FIG. 1D. In FIG. 1E, a mask shape 135 (corresponding toideal circuit shape 100) is formed on a mask substrate 140. A collar 145has been added to mask shape 135 proximate to where mask shape 135 wouldbe aligned over interface 105 on a semiconductor substrate. Collar 145extends from both sides 150 of mask shape 145 and overlaps interface105.

FIG. 1F is a detailed view of FIG. 1E. In FIG. 1F, a reference line 155corresponds to the ideal location of interface 105 (see FIG. 1E)relative to mask shape 135 when a photomask containing mask shape 135 isperfectly aligned to a substrate having interface 105. Mask shape 135has a nominal width W1 and collar 145 has a width W1+2W2. In one exampleW2 is between about 4% and about 12% of W1. In one example W1 less than100 nm. Collar 145 has a length 2L1 where L1 between about 4% and about12% of W1.

FIG. 1G is a top view of an actual integrated illustratingelimination/reduction of the defect to be corrected by the embodimentsof the present invention. FIG. 1G is similar to FIG. 1A and FIG. 1Dexcept an actual circuit shape 100B has a collar region 130A proximateto interface 105. Collar region 130A occurs when a collared mask shapeas shown in FIG. 1E is used. Collar region 130A extends laterally alonginterface 105 a distance D5 over first region 110 and a distance D6 oversecond region 115 on both sides of circuit shape 100B. In one example D5and D6 are about the same. In another example D5 and D6 are different.Collar region 130A extends a distance D7 away from interface 105 overfirst region 110 and a distance D7 and extends a distance D8 away frominterface 105 over second region 115 on both sides of circuit shape100B. In one example D7 and D8 are about the same. In another example D7and D8 are different. As discussed supra, the amount of necking shown inFIG. 1D is variable in that the total amount of necking could vary from0% up to 100% of the circuit shape width across a chip or wafer. Thismeans that the correction shown in FIG. 1E could result in a featurewithout either necking or without a collar (D5=D6=D7=D8=0) as shown inFIG. 1A. If the collar width W2 shown in FIG. 1F is set to eliminateworst-case necking of circuit shapes, then there may be some cases wherea circuit shape having a collar is created as illustrated in FIG. 1G. IfW2 is set to eliminate necking on particular circuit shapes, then theremay be a mixture of circuit shapes on the integrated circuit chip/waferwith varying degrees of necking, varying size collars, and no necking orcollar.

FIG. 2A is a top view illustrating an ideal and a defective multi-fingerfield effect transistor (FET) structure. In FIG. 2A, a multi-finger gateelectrode 200 includes a spine 205 and integral perpendicular fingers210. Fingers 210 extend over a silicon region 215. Silicon region 215 issurrounded by shallow trench isolation (STI) 220. STI 220 comprises aninsulator. Source/drains will be formed in those portions of siliconregion 215 not covered by fingers 210. In one example STI 220 comprisessilicon dioxide. Fingers 210 cross over an interface 225 between siliconregion 215 and STI 220. In one example, a top surface of silicon region215 and a top surface of STI 220 are coplanar but have differentreflectivities to actinic radiation. In one example, the top surface ofsilicon region 215 and the top surface of STI 220 are parallel but notcoplanar and may have the same or different reflectivities to actinicradiation. When the gate electrode mask shape illustrated in FIG. 2B isused to fabricate gate electrode 200, necked regions 230 (dashed lines)are formed in fingers 210 proximate to spine 205. In one example, usingthe gate electrode the mask shape of FIG. 2B, the width of fingers 210over interface 225 is reduced to about 80% of nominal. Necking offingers 210 causes increased leakage, increased stand by power due to Vtroll off and increased Idsat. When the gate electrode mask shapeillustrated in FIG. 2C is used to fabricate gate electrode 200, there isno necking or the necking is greatly reduced. In one example, using thegate electrode the mask shape of FIG. 2C, the width of fingers 210 overinterface 225 is between about 97.5 to about 100% of nominal. In oneexample, gate electrode necking results in about a 30% increased instandby current.

FIG. 2B is a top view of a gate electrode photomask shape correspondingto the

FET of FIG. 2A after model-based OPC. In FIG. 2B model-based OPC hasbeen used to generate a gate electrode mask shape 235 having a spine 240and integral perpendicular fingers 245. The model-based OPC has addedsub-lithographic shapes 250 (boxes overlapping the corners) to distalends 255 of fingers 245. The model-based OPC has also addedsub-lithographic shapes 260 (notches) at internal intersections of spine240 and fingers 245. A sub-lithographic shape is a shape that is toosmall to print in the photolithographic system modeled in the OPCsystem. Reference line 225A indicate where gate electrode shape 235would be perfectly aligned to interface 225 of FIG. 2A preparatory toprinting an image of gate electrode circuit shape 205 in a photoresistlayer on silicon region 215 and STI 220. As noted supra, gate electrodeshape 235 does not correct necking. This is because model-based OPCmodels are (1) based on flat surfaces and (2) cannot look back toprevious mask levels (which define the location of interface 225 (seeFIG. 2A) relative to fingers 210.

FIG. 2C is a top view of a gate electrode photomask shape correspondingto the FET of FIG. 2A after model-based OPC and rule-based OPC accordingto embodiments of the present invention. FIG. 2C is similar to FIG. 2Bexcept collars 265 have been added to fingers 245 over reference line225A. In FIG. 2C, the model-based OPC described in reference to FIG, 2Bhas been performed followed by a rule-based OPC that adds collars 265.Collars 265 are spaced a distance away from spine 240 and do not touchspine 240.

The addition of rule-based OPC is illustrated in FIGS. 3, 4 and 5 anddescribed infra. The differences between FIGS. 3, 4 and 5 is the degreeof automation in generating a collar rule for the rule-based OPC.

FIG. 3 is a flowchart of a first method for compensating for variationsin structures of an integrated circuit. In step 300, model-based OPC isperformed on mask design shapes. In step 305, an initial mask havingonly model-based OPC corrected mask shapes is fabricated. In step 310,circuit shapes are fabricated on an initial integrated circuit using thephotomasks from step 305. The initial integrated circuit has aninterface between first and second regions (e.g., between silicon andinsulator regions). In step 315, selected circuit shapes (e.g.,multi-finger gate electrodes and other circuit shapes found to havenecking problems when perpendicularly crossing a lower level boundaryand which significantly effect the performance of the integratedcircuit) are inspected/measured. This may be a manual inspection or anautomated machine inspection/measurement. In step 320, circuit shapesneeding collars are selected. In step 325, a test mask having sets ofmask shapes corresponding to the selected mask shapes are fabricated,each mask shape of each set of mask shapes having different collarsizes. The test mask itself, may have had model-based OPC correctionapplied. In step 330, test circuit shapes are fabricated on a testintegrated circuit (having a similar interface and similar first andsecond regions as the initial integrated circuit) using the test maskand the collar resulting in a width of the circuit shape over theinterface closet to the nominal width of the circuit shape is selected.Note, some collars may still have some degree of necking and somecollars may result in negative necking (i.e., bulging). In step 335, anOPC rule adding a particular sized collar to particular mask shapes inparticular locations is created. The OPC rule is based on the analysisof step 330 and may be a simple selection of the collar giving a widthclosest to a nominal width (i.e., no necking) or an extrapolationbetween two collars giving widths on opposite sides of the nominalwidth. In step 345, model-based OPC is first applied to a mask designshapes, then a rule-based OPC containing the collar rule(s) is appliedto mask design shapes. In step 350, a final mask having model-based OPCand rule-based OPC corrected mask shapes is fabricated. It isadvantageous that the model-based OPC be performed first, as performingmodel-based OPC after rule-based OPC could adversely affect the finaldimensions of the collars. The final mask is used to fabricatemanufacturing integrated circuits.

FIG. 4 is a flowchart of a second method for compensating for variationsin structures of an integrated circuit. In FIG. 4, steps 400, 405, 410,415 and 420 are the same as respective steps 300, 305, 310, 315 and 320of FIG. 3. In FIG. 425, a software module calculates a collar rule foreach circuit shape needing a collar added to the mask shape. In oneexample, the software module calculates the collar rule-based on a widthof the shape. In one example, the software module calculates the collarrule-based on the degree of necking. Steps 430, 435 and 440 are the sameas respective steps 340, 345 and 350 of FIG. 3.

FIG. 5 is a flowchart of a third method for compensating for variationsin structures of an integrated circuit. In step 450, two or moredifferent shapes files of a same circuit designs are selected. In step455, design shapes in the two or more mask level designs are comparedlooking for perpendicular crossovers of certain design shapes (e.g.,multi-finger gate electrodes perpendicularly crossing asilicon-insulator interface defined by earlier mask levels and othercircuit shapes found to have necking problems when perpendicularlycrossing a lower level boundary and which significantly effect theperformance of the integrated circuit) or in certain regions of a designgrid. The design grid defines the location of design shapes, mask shapesand circuit shapes relative to reference points on the design grid(e.g., a corner of the grid having coordinates 0,0). Steps 460, 465, 470and 475 are the same as respective steps 425, 430, 435 and 440 of FIG.4.

It should be understood that an additional correction may be applied tomask shapes. This correction is called compensation. Compensation isapplied to all mask design shapes in a shapes file and increases ordecreases the width and lengths of all mask design shapes the same.Compensation should not be confused with OPC, which does not adjust allmask design shapes the same. Compensation may be applied immediatelybefore or after steps 330 and 345 of FIG. 3, immediately before or aftersteps 400 and 435 of FIG. 4, and immediately before or after step 470 ofFIG. 5. One purpose of compensation is to adjust for non-lithographicbias of the manufacturing process (e.g., etch bias, out-diffusion, etc).

FIG. 6 is a schematic block diagram of a general-purpose computer forpracticing the present invention. Generally, the method described hereinwith respect to compensating for variations in structures of anintegrated circuit is practiced with a general-purpose computer and themethods described supra in steps 300, 335, 340 and 345 of the flowdiagrams of FIG. 3, steps 400, 425, 430, and 435 of FIG. 4 and steps450, 455, 460, 465 and 470 of FIG. 5 may be coded as a set ofinstructions on removable or hard media for use by the general-purposecomputer.

In FIG. 6, computer system 500 has at least one microprocessor orcentral processing unit (CPU) 505. CPU 505 is interconnected via asystem bus 510 to a random access memory (RAM) 515, a read-only memory(ROM) 520, an input/output (I/O) adapter 525 for a connecting aremovable data and/or program storage device 530 and a mass data and/orprogram storage device 535, a user interface adapter 540 for connectinga keyboard 545 and a mouse 550, a port adapter 555 for connecting a dataport 560 and a display adapter 565 for connecting a display device 570.

ROM 520 contains the basic operating system for computer system 500. Theoperating system may alternatively reside in RAM 515 or elsewhere as isknown in the art. Examples of removable data and/or program storagedevice 630 include magnetic media such as floppy drives and tape drivesand optical media such as CD ROM drives. Examples of mass data and/orprogram storage device 535 include electronic, magnetic, optical,electromagnetic, infrared, and semiconductor devices. Examples of acomputer-readable medium include a semiconductor or solid-state memory,magnetic tape, a removable computer diskette, a random access memory(RAM), a read-only memory (ROM), a rigid magnetic disk and an opticaldisk. Current examples of optical disks include compact disk-read onlymemory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. In additionto keyboard 545 and mouse 550, other user input devices such astrackballs, writing tablets, pressure pads, microphones, light pens andposition-sensing screen displays may be connected to user interface 540.Examples of display devices include cathode-ray tubes (CRT) and liquidcrystal displays (LCD).

A computer program with an appropriate application interface may becreated by one of skill in the art and stored on the system or a dataand/or program storage device to simplify the practicing of thisinvention. In operation, information for or the computer program createdto run the present invention is loaded on the appropriate removable dataand/or program storage device 530, fed through data port 560 or typed inusing keyboard 545.

Thus the present invention discloses a process for supporting computerinfrastructure, integrating, hosting, maintaining, and deployingcomputer-readable code into the computer system 500, wherein the code incombination with the computer system 500 is capable of performing amethod for compensating for variations in structures of an integratedcircuit.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A method, comprising: selecting a mask designshape; selecting a region of said mask design shape; using a computer,applying a model-based optical proximity correction to all regions ofsaid mask design shape, said model optical proximity correction based ona computer model of a photolithographic process that generates asimulated photoresist pattern based on said mask design shape; aftersaid applying said model-based optical proximity correction, using acomputer, applying a rule-based optical proximity correction to saidselected region of said mask design shape, said rule-based opticalproximity correction based on testing for a particular conditionoccurring in a mask design shape and applying a rule to said mask designshape to change the mask design shape.
 2. The method of claim 1, wheresaid mask design shape defines a circuit shape and said circuit shapecrosses an interface between a first region and an abutting secondregion of an integrated circuit.
 3. The method of claim 2, wherein saidcircuit shape crosses said interface perpendicularly.
 4. The method ofclaim 2, wherein said first and second regions have differentreflectivities.
 5. The method of claim 2, wherein a top surface of saidfirst region is raised above a top surface of said second region forminga step.
 6. The method of claim 2, wherein applying said rule-basedoptical proximity correction includes placing a design collar on saidmask design shape in said selected region, said design collar increasinga width of said mask design shape proximate to said interface in adirection parallel to a reference line corresponding in location anddirection to said interface, said design collar extending proximate tosaid reference line perpendicularly on either side of said referenceline.
 7. The method of claim 6, further including before said selectingsaid mask design shape: defining a set of test mask design shapes basedon said mask design shape; placing different size test collars ondifferent test mask design shapes of said set of test mask designshapes; fabricating a test photomask containing a set of test maskshapes from said set of test mask design shapes; fabricating a testintegrated circuit having a test interface between first and second testregions, said test interface between said first and second test regionscorresponding to said interface between said first and second regions;fabricating a set of test circuit shapes corresponding to said set ofmask shapes from said test mask; and selecting collar dimensions forsaid design collar based on measurements of said test circuit shapes inlocations over said test interface.
 8. The method of claim 7, furtherincluding: before placing said different sized test collars, performinga model-based optical proximity correction on each mask design shape ofsaid set of mask design shapes.
 9. The method of claim 6, whereindimensions of said design collar are based on a width of said maskdesign shape measured parallel to said reference line and away from saidreference line.
 10. The method of claim 6, further including: after saidapplying said rule-based optical proximity correction, fabricating amanufacturing photomask having a mask shape having a mask collarcorresponding to said mask design shape having said design collar; andafter said fabricating said manufacturing photomask, fabricating saidcircuit shape using said manufacturing photomask having said mask shapehaving said mask collar, wherein said mask collar on said mask shapecompensates for reflectivity differences between said first and secondregions.
 11. The method of claim 6, further including: after saidapplying said rule-based optical proximity correction, fabricating amanufacturing photomask having a mask shape having a mask collarcorresponding to said mask design shape including said design collar;and after said fabricating said manufacturing photomask, fabricatingsaid circuit shape using said manufacturing photomask having said maskshape having said mask collar, wherein said mask collar on said maskshape prevents necking of said circuit shape over said interface. 12.The method of claim 1, wherein said rule-based optical proximitycorrection is applied only to said selected region of said mask designshape.
 13. The method of claim 1, wherein said mask design shape isselected from a shapes file used to fabricate photomasks for fabricatingan integrated circuit.
 14. A method, comprising: selecting a mask designshape; selecting a regions of said mask design shape, said mask designshape comprising a spine and two or more fingers extendingperpendicularly from said spine, said selected regions consisting ofregions of each of said fingers that are proximate to said spine; usinga computer, applying a model-based optical proximity correction to allregions of said mask design shape, said model-based optical proximitycorrection based on a computer model of a photolithographic process thatgenerates a simulated photoresist pattern based on said mask designshape; and after said applying said model-based optical proximitycorrection, using a computer, applying a rule-based optical proximitycorrection to said selected region of said mask design shape, saidrule-based optical proximity correction based on testing for aparticular condition occurring in a mask design shape and applying arule to said mask design shape to change the mask design shape.
 15. Themethod of claim 14, where said mask design shape defines a multi-fingergate electrode circuit shape and said selected regions of said maskdesign shape define regions of said fingers of said multi-finger gateelectrode circuit shape that cross an interface between a silicon regionand an abutting insulator region of an integrated circuit.
 16. Themethod of claim 14, wherein said fingers cross said interfaceperpendicularly.
 17. The method of claim 14, wherein said silicon regionand said insulator regions have different reflectivities.
 18. The methodof claim 15, wherein a top surface of said silicon region and a topsurface of said insulator region are parallel but not coplanar.
 19. Themethod of claim 14, wherein said applying sais rules based opticalproximity correction includes placing design collars on fingers of saidmask design shape in said selected regions, said design collarsincreasing widths of said fingers of said mask design shape proximate tosaid interface in a direction parallel to a reference line correspondingin location and direction to said interface, said design collarsextending proximate to said reference line perpendicularly on eitherside of said reference line, said collars spaced away from said spine.20. The method of claim 19, further including before said selecting saidmask design shape: defining a set of test mask design shapes based onsaid mask design shape; placing different size test collars on differenttest mask design shapes of said set of test mask design shapes;fabricating a test photomask containing a set of test mask shapes fromsaid set of test mask design shapes; fabricating a test integratedcircuit having a test interface between test silicon and test insulatorregions, said test interface between said test silicon and testinsulator regions corresponding to said interface between said siliconand insulator regions; fabricating a set of test circuit shapescorresponding to said set of mask shapes from said test mask; andselecting collar dimensions for said design collars based onmeasurements of said test circuit shapes in locations over said testinterface.
 21. The method of claim 20, further including: before placingsaid different sized test collars, performing a model-based opticalproximity correction on each mask design shape of said set of maskdesign shapes.
 22. The method of claim 19, wherein dimensions of saiddesign collars are based on a width of said fingers of said mask designshape measured parallel to said reference line and away from saidreference line.
 23. The method of claim 19, further including: aftersaid applying said rules based proximity correction, fabricating amanufacturing photomask having a mask shape including mask collarscorresponding to said mask design shape having said design collars; andafter said fabricating said manufacturing photomask, fabricating saidcircuit shape using said manufacturing photomask, wherein said maskcollars on said fingers of said mask shape compensate for reflectivitydifferences between said silicon and insulator regions.
 24. The methodof claim 19, further including: after applying said rules based opticalproximity correction, fabricating a manufacturing photomask having amask shape including mask collars corresponding to said mask designshape including said design collars; and after said fabricating saidmanufacturing photomask, fabricating said circuit shape using saidmanufacturing photomask, wherein said mask collars on said mask shapeprevent necking of said fingers of said circuit shape over saidinterface.
 25. The method of claim 14, wherein said mask design shape isselected from a shapes file used to fabricate photomasks for fabricatingan integrated circuit.
 26. The method of claim 14, wherein said secondoptical proximity correction is applied only to said selected region ofsaid mask design shape.